Mosquitoes (Diptera:  Culicidae ) are vectors of vector-borne infections in South Vietnam (materials of the spring expedition 2024)

Roman V. Gudkov; Гудков Роман Владимирович; Roman V. Gudkov; Aleksey I. Solovyov; Соловьев Алексей Иванович; Aleksey I. Solovyov; Konstantin V. Kozlov; Козлов Константин Вадимович; Konstantin V. Kozlov; Dmitriy V. Ovchinnikov; Овчинников Дмитрий Валерьевич; Dmitriy V. Ovchinnikov; Oleg V. Maltsev; Мальцев Олег Вениаминович; Oleg V. Maltsev; Vitaliy S. Sukachev; Сукачев Виталий Сергеевич; Vitaliy S. Sukachev; Artem R. Ariukov; Арюков Артем Русланович; Artem R. Ariukov; Vladimir A. Romanenko; Романенко Владимир Александрович; Vladimir A. Romanenko; Aleksandr I. Rakin; Ракин Александр Ильич; Aleksandr I. Rakin; Mo Thi  Luong; Лыонг Мо Тхи; Thi Luong; Nam Van Thanh Nguyen; Нгуен Нам Ван Тхань; Nam Van Thanh Nguyen; Truong Van  Tran; Чан Чыонг Ван; Truong Van  Tran; Hiep Van  Nguyen; Нгуен Хиеп Ван; Hiep Van  Nguyen; Bien Van  Fan; Фан Биен Ван; Bien Van  Fan

doi:10.17816/rmmar637084

Mosquitoes (Diptera: Culicidae) are vectors of vector-borne infections in South Vietnam (materials of the spring expedition 2024)

Authors: Gudkov R.V.¹, Solovyov A.I.¹, Kozlov K.V.¹, Ovchinnikov D.V.¹, Maltsev O.V.¹, Sukachev V.S.¹, Ariukov A.R.¹, Romanenko V.A.¹, Rakin A.I.¹, Luong M.T.², Nguyen N.V.², Tran T.V.², Nguyen H.V.², Fan B.V.³
Affiliations:
1. Military Medical Academy
2. Joint Russian-Vietnamese Tropical Research and Technological Center, Southern Branch
3. Bù Gia Mâp National Park
Issue: Vol 43, No 4 (2024)
Pages: 447-455
Section: Original articles
URL: https://journals.eco-vector.com/RMMArep/article/view/637084
DOI: https://doi.org/10.17816/rmmar637084
ID: 637084

Cite item

Full Text

Abstract
Full Text
About the authors
References
Supplementary files
Statistics

Abstract

BACKGROUND: Blood-sucking mosquitoes are carriers of many human vector-borne diseases caused by viruses, bacteria, protozoa and helminths. There are the results of entomological studies of the Joint Russian-Vietnamese Tropical Research and Technology Center.

AIM: is to study the generic and species composition of mosquitoes in South Vietnam before the rainy season, when mosquito activity is low.

MATERIALS AND METHODS: The collection of arthropods was carried out from May 12 to 26, 2024 in two regions of South Vietnam: the coastal mangrove reserve in the Can Gio area of Ho Chi Minh City, as well as in the Bu Gia Map Reserve of Binh Phuoc province. Imago was collected by exhusters “on themselves” and from feeders, entomological nets from plants, as well as inside and outside residential and outbuildings. The larvae were collected by filtering water samples from natural and artificial reservoirs where mosquitoes lay eggs. Arthropods were identified by morphological features.

RESULTS: The analysis of the collected material in two regions of South Vietnam showed that before the rainy season, mosquitoes of the genus Culex predominate 90.7%, mosquitoes of the genus Anopheles and Aedes account for 6.1% and 3.2%, respectively.

CONCLUSION: Thus, in South Vietnam, at the end of the “dry season”, mosquitoes of the genus Culex predominate, adapted to development in conditions of reduced breeding sites. The study of the mechanisms of functioning of parasitic systems involving blood-sucking mosquitoes requires further research with the expansion of the research area. It is advisable to conduct entomological monitoring in various landscape and climatic zones during different periods of the epidemic season.

Keywords

Aedes, Anopheles, blood-sucking mosquitoes, Culex, Culicidae, Malaria, South Vietnam

Full Text

BACKGROUND

Mosquitoes are widespread dipterous insects of the family Culicidae, with over 3,500 species [1]. Adult mosquitoes have one pair of wings, three pairs of legs, one pair of antennas, feelers, a narrow segmented abdomen, and piercing-sucking mouthparts. Mosquitoes are insects with holometabolous development, which includes the egg, larva, pupa, and imago stages. Females need protein for the eggs to develop, which they receive through blood sucking from mammals, birds, reptiles, amphibians, and other animals [1, 2].

Bloodsucking mosquitoes are carriers of numerous vector-borne human diseases caused by viruses, bacteria, protozoa, and parasitic worms [3]. The main groups of mosquitoes differ by the ability to transmit specific pathogens. For example, mosquitoes of the genus Anopheles are specific vectors for malaria, wuchereriasis, Malayan filariasis, and o’nyong-nyong fever pathogens. Mosquitoes of the genus Aedes are vectors for dengue fever, chikungunya, Zika, Rift Valley fever, and yellow fever viruses, as well as other tropical disease pathogens. Mosquitoes of the genus Culex play a critical role in the transmission of socially significant diseases such as Japanese encephalitis and West Nile fever [4]. Mosquito-transmitted diseases have a broad nosorange, with tropical and subtropical regions being hyperendemic for some of these dangerous infections [5]. According to the World Health Organization (WHO), approximately 219 million cases of malaria are reported globally each year, with more than 445 thousand fatal outcomes. The annual incidence of dengue fever is 96 million cases, with approximately 40 thousand fatal outcomes [6]. There are no vaccines against the majority of mosquito-transmitted diseases. Thus, vector control is one of the most important measures of vector-borne disease control [7].

Vietnam is a tropical country in Southeast Asia with a high risk of mosquito-transmitted diseases [8]. Despite extensive malaria control measures, the disease has not been fully eliminated in Vietnam. According to the WHO Regional Office for the Western Pacific, Vietnam is currently one of seven countries (Australia, Cambodia, China, Laos, Malaysia, Singapore, and Vietnam) with the highest incidence of dengue fever. According to the Ministry of Health of Vietnam, the incidence of dengue fever in 2023 was 171,585 cases, with 42 fatal outcomes; in 2022, the incidence was 382,458 cases, with 115 fatal outcomes. In the first quarter of 2024, there were approximately 16 thousand patients with dengue fever, with one fatal outcome.

This study was performed in accordance with the research plan of the Joint Russian-Vietnamese Tropical Research and Technological Center, as part of the projects “Phase-related (seasonal) genetic changes in Plasmodium falciparum virulence and drug resistance in the body of vectors: mosquitoes of the genus Anopheles” and “Functional patterns of biological systems involving flaviviruses in Southeast Asia.” During the first stage of the research, it was planned to visit vector-borne infection centers in Vietnam and collect bloodsucking vector samples. The study provided for testing the methods of collection, storage, and morphological identification of insects for biological sample preparation for subsequent molecular genetic testing.

Study aim: To perform a preliminary assessment of the genus and species composition of vectors for the most relevant socially significant vector-borne infections in Southern Vietnam during the low mosquito activity period preceding the wet season.

MATERIALS AND METHODS

The insects were collected on May 12–26, 2024, by a joint research team, which included researchers from the Military Medical Academy and the Southern Department of the Russian-Vietnamese Tropical Research and Technological Center (Table 2; Figure 1, a). The studies were performed in two regions of Southern Vietnam. The first study region was Can Gio district (Ho Chi Minh City, 10°27'17.588''N 106°53'30.669''E). The district is located in a coastal lowland, at the mouth of the Saigon and Dong Nai rivers, within the Can Gio Mangrove Biosphere Reserve. Mosquito breeding areas and adult insect habitats around the Tropical Center station were examined (Figure 1, b, c). The insects were collected from coastal flooded mangrove forests, artificial water bodies (irrigation canals, mollusc hatcheries), residential and household buildings (houses, barns, etc.), and natural and artificial fresh rainwater reservoirs.

Table 1. Characteristics of the entomological studies carried out and the volume of activities carried out

Таблица 1. Характеристика проведенных энтомологических исследований и объем выполненных мероприятий

Study methods and procedures	Can Gio	Bu Gia Map
Determination of the species composition of malaria mosquitoes	887	185
Determination of the daily activity pattern of malaria mosquitoes by human landing catches	25	19
Examination of control daytime rest sites of malaria mosquitoes	25	9
Assessment of the number of vectors in military camps	54	36
Examination of anophelogenous water bodies	46	12

Fig. 1. Places of collection of vectors and malarial landscapes; а — district of mosquito collection: 1 — Can Gio, coastal district of Ho Chi Minh City; 2 — Bu Gia Map district, Binh Phuoc province; б — agricultural irrigation channels, Kang Zyo; в — flooded mangrove forests of the coast; г — hilly area with a river network, Uezd Bu Za Map

Рис. 1. Места проведения сбора переносчиков и маляриогенные ландшафты; а — районы сбора комаров: 1 — Кан Зьо (Can Gio), прибрежный район города Хошимин; 2 — уезд Бу За Мап (Bu Gia Map), провинция Бинь Фыок (Binh Phuoc); б — сельскохозяйственные оросительные каналы, Кан Зьо; в — затопляемые мангровые леса побережья; г — холмистый район с речной сетью, Уезд Бу За Мап

The second study region was the Bu Gia Map National Park and the neighboring settlement in the Bu Gia Map district of the Binh Phuoc province (12°5'22.878''N 107°9'26.866''E, 310 m above sea level). Hillside woodlands with rivers and small lakes, as well as infrequent single-story buildings with various water tanks and surrounding rubber tree plantations, were examined (Figure 1, d). The settlement is likewise largely made up of single-story houses with household buildings and water collection and storage tanks. Some residents keep livestock and poultry (buffalos, pigs, chickens).

Adult mosquitoes were collected outdoors in the evening, during their peak bloodsucking activity, using small nets with a diameter of 20 cm. Moreover, the insects were collected from hosts using exhausters, which were also used to remove adult insects from nets (Figure 2, a). To collect flying mosquitoes, ultraviolet light traps were installed overnight near livestock barns and residential buildings (Figure 2, b). Endophagous mosquitoes in residential buildings were collected from windows using an exhauster and a special killing bottle.

Fig. 2. Methods of collecting imago: а — catching using exhausters and nets; b — light ultraviolet trap

Рис. 2. Методы сбора имаго: а — отлов с использованием эксгаустеров и сачков; b — световая ультрафиолетовая ловушка

Female mosquitoes were collected from daytime rest sites on the walls and ceilings of residential and household buildings, as well as from plants near the houses.

Larvae were collected from nearby natural and artificial water bodies suitable for mosquito breeding (swamps in mangrove forests, tree hollows, puddles, irrigation canals, mollusc hatcheries, rainwater reservoirs, discarded containers filled with water, etc.) (Figure 3, a). Mobile larvae were collected using a ladle; any extra water was filtered through a sieve (Figure 3, b). The collected larvae were placed in 20–50 mL transport containers with a small amount of water.

Fig. 3. Collection of mosquito larvae: а — artificial house pond with larvae; b — use of a ladle and strainer to filter water with larvae from a house well

Рис. 3. Сбор личинок комаров: а — искусственный придомовой водоем с личинками; b — использование ковша и ситечка для фильтрации воды с личинками из придомового колодца

Mosquitos caught with exhausters and traps were treated with ethyl acetate. Following suffocation, immobilized insects were placed on a light surface or in a small container for sorting (Figure 4, a).

Fig. 4. Morphological identification and storage of collected material: а — pickling of captured imagoes with ethyl acetate with subsequent identification; b — sorting and marking of collected larvae

Рис. 4. Морфологическая идентификация и хранение собранного материала: а — замаривание этилацетатом отловленных имаго с последующей идентификацией; b — сортировка и маркировка собранных личинок

To preserve the morphological characteristics during sorting, special entomological tools (tweezers, preparation needles, etc.) were used. The selected insects were placed into special boxes with a cotton wool layer. Samples intended for subsequent molecular genetic testing were preserved in test tubes with 70% ethanol. The larvae were preserved in alcohol (Figure 4, b). Entomological magnifiers 10x and 20x were used to identify mosquitos in the field. Standard methods were used to identify the genus based on morphological characteristics.

RESULTS AND DISCUSSION

The research team collected bloodsucking mosquitoes on May 15–18, 2024, in the coastal district Can Gio (Figure 5, a–d). Table 2 shows the current genus composition of mosquitoes.

Fig. 5. Survey of residential buildings in the Can Gio area: а — collection of larvae from rainwater tanks; b — survey of areas with standing water; c — adults during the day; d — plants in the local area with mosquitoes on the leaves

Рис. 5. Обследование жилых домов в районе Кан Зьо (Can Gio): а — сбор личинок из резервуаров с дождевой водой; b — обследование мест со стоячей водой; c — имаго на дневке; d — растения на придомовой территории с комарами на листве

Table 2. Distribution of vector mosquitoes collected in the coastal area of Kan Zyo by genera and stages of development

Таблица 2. Распределение комаров-переносчиков, собранных в прибрежном районе Кан Зьо по родам и стадиям развития

Mosquito development stages	Collected mosquitoes by development stage	Mosquito genera
Mosquito development stages	Collected mosquitoes by development stage	Culex	Anopheles	Aedes
Imago	283/54.6%	217/76.7%	54/19.1%	12/4.2%
Larva	236/45.4%	231/97.8%	5/2.2%	–
Total	519/100%	448/86.3%	59/11.4%	12/2.3%

Adult insects accounted for 54.6% of all collected mosquitoes. Culex was the most common genus (76.7%), followed by Anopheles (19.1%) and Aedes (4.2%). Aedes larvae were not found, while Culex larvae were the most commonly detected. Mosquitoes of the genus Anopheles at various development stages were most likely more prevalent than Aedes (11.4% vs. 2.3%) due to the greatest number of suitable breeding areas in the examined region.

In total, 736 samples were collected over four days in the Bu Gia Map National Park and the neighboring settlement (Figure 6, a–f; Table 3).

Fig. 6. Survey of residential buildings in the Bu Gia Map Nature Reserve and the adjacent commune: а — collection of mosquitoes in houses in the reserve; b — study of a tree hollow with stagnant water; c — buffaloes that feed blood-sucking mosquitoes; d — collection of larvae from a container with rainwater in the backyard; e, f — inspection of yards and houses in the commune, collection of mosquitoes during the day

Рис. 6. Обследование жилых домов на территории заповедника Бу За Мап (Bu Gia Map) и прилегающей коммуне: а — сбор комаров в домах на территории заповедника; b — исследование дупла дерева со стоячей водой; c — буйволы-прокормители кровососущих комаров; d — сбор личинок из емкости с дождевой водой на заднем дворе; e, f — обход дворов и домов в коммуне, сбор комаров на дневках

Table 3. Distribution of vector mosquitoes collected in Bu County for Map by birth and stage of development

Таблица 3. Распределение комаров-переносчиков, собранных в уезде Бу За Мап, по родам и стадиям развития

Mosquito development stages	Collected mosquitoes by development stage	Mosquito genera
Mosquito development stages	Collected mosquitoes by development stage	Culex	Anopheles	Aedes
Imago	463/62.9%	423/91.4%	12/2.6%	28/6.0%
Larva	273/37.1%	267/97.8%	6/2.2%	–
Total	736/100%	690/93.8%	18/2.4%	28/3.8%

Unlike the first region, there were no significant differences in the proportion of collected insects between Anopheles and Aedes (2.4% vs. 3.8%), with Culex being the most prevalent genus (93.8%).

The analysis of materials collected in two regions of Southern Vietnam revealed that mosquitoes of the genus Culex were the most prevalent carriers of vector-borne disease pathogens (90.7%) before the start of the wet season, while Anopheles and Aedes accounted for 6.1% and 3.2%, respectively (Table 4). The proportion of mosquitoes of the genera Anopheles and Aedes will probably increase when the high-intensity transmission season begins.

Table 4. Distribution of vector mosquitoes by birth, by birth and stage of development collected during the entire expedition

Таблица 4. Распределение комаров-переносчиков собранных за все время экспедиции по родам и стадиям развития

Mosquito development stages	Collected mosquitoes by development stage	Mosquito genera
Mosquito development stages	Collected mosquitoes by development stage	Culex	Anopheles	Aedes
Imago	746/59.4%	640/85.8%	66/8.8%	40/5.4%
Larva	509/40.6%	498/97.8%	11/2.2%	–
Total	1255/100%	1138/90.7%	77/6.1%	40/3.2%

However, it should be noted that our data only cover a limited period of the epidemic season. Thus, they do not represent the annual genus and species composition distribution of mosquitoes transmitting vector-borne infections.

CONCLUSION

The study findings indicate that the species composition of vectors at the end of the dry season in Southern Vietnam was dominated by mosquitoes of the genus Culex, which are well-adapted to development and population maintenance with limited areas suitable for breeding. The functional mechanisms of parasitic systems involving bloodsucking mosquitoes requires further research with expanded study areas. Entomological monitoring under various terrain and climatic conditions throughout the transmission season of vector-borne disease pathogens will be beneficial.

ADDITIONAL INFO

Authors’ contribution. All authors made a substantial contribution to the conception of the study, acquisition, analysis, interpretation of data for the work, drafting and revising the article, final approval of the version to be published and agree to be accountable for all aspects of the study.

Funding source. The study was not supported by any external sources of funding.

Consent for publication. Written consent was obtained from the patients for publication of relevant medical information within the manuscript.

ДОПОЛНИТЕЛЬНАЯ ИНФОРМАЦИЯ

Вклад авторов. Все авторы внесли существенный вклад в проведение исследования и подготовку статьи, прочли и одобрили финальную версию перед публикацией.

Финансирование. Поисково-аналитическая работа проведена на личные средства авторского коллектива.

Конфликт интересов. Авторы декларируют отсутствие явных и потенциальных конфликтов интересов, связанных с публикацией настоящей статьи.

About the authors

Roman V. Gudkov

Military Medical Academy

Author for correspondence.
Email: gudkoff@mail.ru
ORCID iD: 0000-0001-5498-0479
SPIN-code: 8311-6296
Scopus Author ID: 57204178016
ResearcherId: L-6478-2016

MD, Cand. Sci. (Medicine), Associate Professor of the Department of Infectious Diseases (with a course in medical parasitology and tropical diseases)